1. Field of the Invention
The invention relates generally to land surveying devices and more specifically to global positioning system based apparatus for land surveying.
2. Description of the Prior Art
Land surveyors conventionally use prisms mounted to poles for targeting by a total station. A typical pole is about five feet long. The pole is held over a target mark on the ground and must be held plumb by a surveyor. Otherwise, an error will be made, because the calculations of the target mark position depend on knowing the length of the pole and on having the prism positioned directly over the mark. The pole is usually plumbed with the aid of a bubble level in a circular vial. An off-plumb condition of the pole could be corrected for with calculations and the correct position of the mark nevertheless determined if the degree of tilt and the direction of tilt were known and included in the calculations. Ordinarily, ascertaining the degree of tilt and its direction would be clumsy and including this information in the calculation of the mark position would be difficult. It is far easier just to hold the pole plumb.
Global positioning system (GPS) receivers have been finding their way into land surveying uses, especially those units that are accurate to a fraction of a centimeter. The GPS receivers use signals received from typically four or more overhead satellites to determine navigational data such as position and velocity. Such systems may also provide altitude and time. GPS signals are available worldwide at no cost and can be used to determine the location of a vehicle, such as a car or truck, to within one city block, or better. Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with the GPS satellites, to generate accumulated delta-range measurements (ADR) from P-code modulation on those carriers and at the same time track L1 coarse acquisition code (C/A-code) to generate code phase measurements. Carrier L1 has a frequency of 1575.42 MHz and carrier L2 has a frequency of 1227.78 MHz.
A number of specific survey applications require the ability to accurately locate existing physical position marks and generate new physical position marks from pre-selected locations contained in a database. These include the marking of positions used in construction and building sites, referred to as stake-out. The pre-selected positions would typically exist on the construction plans. The accuracy of the generated position marks must typically be to within five millimeters to three centimeters.
Traditional techniques for generating physical position marks from a mapping database rely on optical instruments such as theodolites and EDM (electronic distance measurement) devices. A more recent survey device is the Total Station that combines a theodolite and an EDM device. A disadvantage of such systems is the necessity for clear visibility between a reference mark, and the new position mark. Without such visibility, multiple measurements may be necessary, which may result in the accumulation of errors.
GPS receivers, or simply their antennas, may be mounted to a land surveyor's pole, but the problem persists in the prior art that such poles must be held plumb. It is, however, human nature not to be unerringly careful about such things. So once in a while such GPS-equipped poles are not held plumb while the measurement is taken. Since desired accuracies in land surveying are typically in the range of five millimeters to one centimeter, even a small amount of range pole tilt can be significant. For example, even a careful manual plumbing of the range pole can result in the top of the pole being as much as five centimeters out of vertical from the bottom.